In recent years, for example, highly confidential information such as My Number (individual number) is increasing, and such information is thought to increase even more in the future. In Japan, My number is a social number issued for each individual based on the Social Security and Tax Number System. Various types of information related to an individual is associated with his/her My Number. Therefore, My Number is confidential information that any leakage thereof ought to be avoided. Accordingly, in the future, it is predicted that there will be a demand for technologies for keeping confidential information secret, with confidentiality even higher than that of conventional technologies. Generally, when a file including confidential information is kept secret, it is conceived that the entire file is encrypted.
However, when a file including confidential information is used, there are cases where displaying and writing of the confidential information are not necessary. For example, when a withholding certificate is used as an income verification, due to the law, his or her My Number is not written on the withholding certificate. In this case, the information of the My Number does not need to be decrypted, and the risk of information leakage is increased if such information is decrypted.
Further, in a case where the entire file including confidential information of a plurality of users is encrypted, when confidential information of any of the users is used, the entire file needs to be decrypted, so that the risk of information leakage is increased. For example, in a case where the entire file including confidential information of one hundred users is encrypted, when confidential information of only one specific user is used, confidential information of the rest of ninety-nine users is also decrypted, so that the risk of information leakage is increased.